Removing default for volume-plugins mountpoint (#2618)

All checks test if this is defined meaning there is no way to undefine it.

roles/kubernetes/node/templates/kubelet.host.service.j2

@@ -7,9 +7,7 @@ Wants=docker.socket
 [Service]
 User=root
 EnvironmentFile=-{{kube_config_dir}}/kubelet.env
-{% if kubelet_flexvolumes_plugins_dir is defined %}
 ExecStartPre=-/bin/mkdir -p {{ kubelet_flexvolumes_plugins_dir }}
-{% endif %}
 ExecStart={{ bin_dir }}/kubelet \
 		$KUBE_LOGTOSTDERR \
 		$KUBE_LOG_LEVEL \